Instructions

Mount this on the four threaded rods beneath the print bed , off center so that the pulley is centered under the bed.

On the bottom two threaded rods just use nuts and optionally washers, on the upper two, use:
Nut->Fender washer->washer>608 bearing->washer->mount->(washer)->nut.
I actually used two 608 bearings to give a large surface for the belt to ride over without rubbing into the fender washer.

Use the thing linked above to attach the belt to your printbed. I altered the assembly and shimmed so that the belt exit point was level with the top of the 608 bearings. I felt this was necessary so that steps per mm wouldnt get skewed closer to the belt tensioners like if the belt was held against the print surface.

This also eliminated problems i was having with the bed stuttering. My printrbot was custom built and modified to fit a prusa heatbed. Doing that made the side mounted y cause the bed to not move smoothly. Prior to this part I had y steppers on both sides of the bed (one wired reverse) to help.